Local anesthetics injected into the body by hand-held syringes to enable otherwise painful medical procedures to be performed without pain have been used for over 100 years. The lesser but nevertheless real pain caused by the syringes persisted until 1986 when the applicant invented an apparatus and method to eliminate it. Such are disclosed in the applicant's U.S. Pat. Nos. 4,747,824 and 5,180,371, which are incorporated herein by reference for their showing of the prior art as it exists today.
The applicant's prior invention provides a computer controlled power driven pump for feeding anesthetic through the needle at, selectively, either one of two different, pre-programmed rates of flow. The lower flow rate is empirically determined to be at or below the maximum rate at which, for the average patient, the anesthetic is absorbed into most tissue without pain. The low flow rate is started at the time of implanting the needle and is continued for a period of time which the doctor estimates to be necessary to pre-numb the surrounding tissues sufficiently to receive the high flow rate without pain.
The high flow rate then drives the anesthetic to the desired nerve target within a reasonable time. The total time may or may not be greater than that taken by the conventional hand syringe injection but the trade off is freedom from pain and a much more pleasant procedure for the doctor and patient alike.
The injection, however, of local anesthetic by any of the prior art methods and apparatus presents a dilemma to the doctor. Whereas a low flow rate for anesthetic is painless, the time the syringe, in the hand of the doctor, remains with the needle implanted in the patient can be a tiring ordeal for the patient and doctor alike, even if the ultimate objective of complete numbing is achieved. Also, a slow rate of injection can defeat the ultimate objective of effective numbing of a body part and can saturate soft tissues to the extent of injury before the nerve is anesthetized. On the other hand, more rapid injection rates translate into higher pressure which instantly translates into pain for the patient. These facts, coupled with the fact that different patients have differing thresholds of pain, and the fact that even the same patient can have differing sensitivity in different injection sites, spell trouble for the doctor. Optimizing the numbing procedure without pain is virtually impossible using all of the known prior art techniques.
Accordingly, the present invention abandons the applicant's basic prior art concept of providing the doctor with a selection of two different, pre-determined flow rates, high and low, which are chosen by a double acting selector switch on a somewhat arbitrary time schedule. First, the double pole selector switch concept is not user friendly for the doctor because it introduces the possibility of human error. Just as important, the prior art introduces a subjective decision by the doctor as to when he dares to select the high flow rate. He faces the dilemma of pain for the patient versus wasted time, poor numbing results, and tissue damage.
An object of the present invention, therefore, is to provide for the injection of local anesthetic in a user friendly manner, in less time and often in lesser amounts to achieve the same degree of numbing, all without discomfort for the patient.
Another object of the invention is to provide apparatus and method which achieve a consistently effective intraligamentary or "PDL" injection of local anesthetic in dentistry which is substantially free of pain and tissue damage for the patient and which, for the benefit of the doctor and patient alike, can often be accomplished in much less time using less anesthetic and with a very low failure rate.
Another object of the invention is to provide an apparatus and method for the painless injection of local anesthetic in which, at any time in the procedure, the patient can arrest the rate of flow of the anesthetic and instantly reduce the pressure of the anesthetic in the tissues and, therefore, preclude pain.
Another object of the invention is to provide an apparatus and method for the painless injection of local anesthetic in which the pressure of the anesthetic at the needle tip within the patient is measured and utilized in the injection process.
Another object of the invention is to provide an improved disposable needle assembly for conveying the anesthetic from the reservoir into the tissue.